I was researching heartworm in my area and found some info off the Heartworm Society page about when and how often to give preventatives. It makes sensd to me and I'm thinking that I'm going to switch the dosing based on the climate. They used Chicago/Illinois as an example so it's spelled out pretty well and I can't screw it up I'm slightly hesitant about 6 week intervals instead of a month but that makes sense to. I've also checked the average nightly temperatures and it is under 64 degrees in May, I haven't checked June, but that is where the 30 day after season would come in making the first dose July 1st. It also states that milk thistle should be given for a week after each dose, but how much of a dose do the dogs get of that? I'm not familiar with milk thistle at all. I've attached the article and opinions are more then welcome.